Apparatus for growing crystals in plate form

ABSTRACT

APPARATUS FOR GROWING CRYSTALS OF SUCH MATERIALS AS SAPPHIRE, RUBY, SPINEL AND THE LIKE IN STRIP OF PLATE FORM BY THE VERNEUIL METHOD. A PLURALITY OF SPACED POWDER FEED TUBES, HAVING MEANS SURROUNDING EACH ONE TO SUPPLY A COMBUSTIBLE MIXTURE, ARE POSITIONED TO DIRECT FUSED   PARTICLES ONTO A BOULE TO BUILD UP THE CRYSTAL IN AN EVEN CONTINUOUS GROWTH PATTERN.

APPARATUS FOR GROWING CRYSTALS IN PLATE FORM Filed Feb. 7, 1969 Oct. 10, 1972 J, wENcKus 4 Sheets-Sheet 1 INVENTOR.

Joseph F. Wenckus Attorney Oct. 10, 1972 v J. F. wENcKus 3,697,228 APPARATUS FOR GROWING CRYSTALS IN PLATE FORM Filed Feb. 7, 1969 4 Sheets-Sheet 2 Fig. 3

Fig. 2

INVENTOR. Joseph F. Wenckus Attorney Oct. 10, 1972 J. F. WENCKUS 3,697,228

APPARATUS FOR GROWING CRYSTALS IN PLATE FORM Filed Feb. 7. 1969 4 Sheets-Sheet 3 Fig. 5

INVENTOR.

Joseph F. Wenckus Attorney Oct. 10, 1972 J. F. WENCKUS 3,697,228

APPARATUS FOR GROWING CRYSTALS IN PLATE FORM Filed Feb. 7, 1969 4 Sheets-Sheet 4 Attorney United States Patent Ofice Patented Oct. 10, 1972 3,697,228 APPARATUS FOR GROWING CRYSTALS IN PLATE FORM Joseph F. Wenckus, Needham, Mass., assignor to Arthur D. Little, Inc., Cambridge, Mass. Filed Feb. 7, 1969, Ser. No. 797,556 Int. Cl. B01j 17/24 US. Cl. 23-273 V 6 Claims ABSTRACT OF THE DISCLOSURE Apparatus for growing crystals of such materials as sapphire, ruby, spinel and the like in strip or plate form by the Verneuil method. A plurality of spaced powder feed tubes, having means surrounding each one to supply a combustible mixture, are positioned to direct fused particles onto a boule to build up the crystal in an even continuous growth pattern.

This invention relates to an apparatus for growing single crystals of inorganic materials and more particularly to the growing of such crystals in a configuration which permits relatively large sheets or plates to be out therefrom.

Intensive investigations of a number of inorganic crystalline materials such as A1 BeO, MgO-Al O Eco-A1 0 and MgO have indicated that these materials possess exceedingly high tensile strength and that they,

therefore, offer the possibility of forming transparent windows for a number of high pressure and/ or high temperature devices. Thus plates of such materials may be incorporated in high-temperature furnaces, used as optical armour and provide windows for periscopes and reentry vehicles.

Much effort has been expended in recent years to develop methods and apparatus for growing single crystals of such inorganic materials for lasers. There is available a large body of knowledge concerning methods for forming these crystals in rod form. These methods include such techniques as chemical-vapor deposition, drawing from a molten pool, and introducing powdered material into a flame for fusion of the material prior to its deposition on a boule. Generally, the different methods include four basic processes, namely Czochralski, Ribbon, Bridgeman and Verneuil. Torches for growing crystals by the Verneuil process have been designed. See for example US. Pat. 3,224,840.

However, methods which are based on drawing crystals from a melt are relatively inflexible with regard to the shapes which can be produced, particularly when it is desired to form relatively thin plates or sheets. Any method based upon chemical-vapor deposition has the potential advantage that it can possibly lend itself to complex shapes, but the process is relatively slow and not well developed to take advantage of this feature.

The Verneuil method, which is a flame fusion technique, appears to offer an economical, reliable and relatively rapid process for growing single crystals in sheet or plate form. However, the two devices known to be described in the prior art for this purpose have so far apparently failed to realize all of the advantages which might be inherent in this method. The one apparatus known to be in use at present is that described in US. Pat. 2,852,890. The apparatus is designed for making large-diameter boules or disks using a horizontally mounted, rotary-arbor modification of the basic flamefusion technique. The resulting disk-shaped boules produced by this method are highly stressed and breakage of the crystal disk during formation is a relatively common occurrence. Moreover, machining of these disks to cut and polish them is extremely difficult, inasmuch as the tensions and stresses formed in the disks during manufacture lead to an extremely high breakage rate during final processing, even after a lengthy annealing operation. Thus, plates or sheets formed from the disk boule grown in this manner are extremely expensive.

The other known disclosure in the prior art is French Pat. 1,269,105 published in 1961. This patent describes an apparatus comprising a multiplicity of alternating powder feed tubes and torches aligned in series which deliver fused powder to a boule support made to oscillate in a horizontal plane. The mechanism is complicated and if the oscillation is not carried out in an extremely precise manner the resulting crystal is uneven and unsatisfactory. It is also possible that the oscillatory motion required forms strains within the crystal due to the transient nature of the temperature gradients experienced during growth.

It would, therefore, be desirable to have an apparatus capable of economically growing inorganic single crystals by the Verneuil method in a configuration which lends itself to cutting and polishing to form relatively large sheets or plates of the inorganic crystal.

It is therefore a primary object of this invention to provide an apparatus which is capable of forming single crystals in sheet or platelike form of inorganic materials which lend themselves to crystal growth by the Verneuil method. It is another object of this invention to provide apparatus of the character described which is reliable and relatively rapid in operation and builds crystals in which the internal stresses and strains are minimized. It is another object of this invention to provide such apparatus which grows single crystals in sheet or plate form which may be machined, polished and otherwise processed without experiencing any appreciable loss of crystal blanks through cracking, chipping, etc. It is another object of this invention to provide apparatus of the character described which is flexible in design and which thus permits formation of crystals in sheet or plate form over a wide size range. It is still another object of this invention to provide such apparatus which eliminates all but one moving component thus minimizing unwanted growth variables. It is another primary object of this invention to provide a method and apparatus for forming single-crystal sheets or plates at a price level which permits much wider use than heretofore feasible. Other objects of the invention will in part be obvious and will in part be apparent hereinafter The invention accordingly comprises the features of construction, combinations of elements and arrangement of parts which will be exemplified in the constructions hereinafter set forth and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention reference should be had to the following detailed description taken in connection with the accompanying drawings in which FIG. 1 is a perspective view, partially cut away, of the apparatus of this invention;

FIG. 2 is a cross section of one embodiment of a torch and its associated powder feed mechanism suitable for the apparatus of this invention;

FIG. 3 is a cross section of the torch and powder feed mechanism of FIG. 2 taken along line 3-3 of FIG. 2; FIG. 4 is a bottom end view of the torch of FIG. 2;

FIG. 5 is a bottom end view of the torch of FIG. 2 illustrating a modified tube arrangement;

FIG. 6 is another embodiment of a torch with the powder feed mechanism suitable for the apparatus of this invention;

FIG. 7 is a cross section of the bottom portion of the torch of FIG. 6 taken along line 7-7 of that figure; and

FIG. 8 is a diagrammatic view of an assembly of a plurality of the apparatus of this invention adapted for growing wide sheets or plates.

In the apparatus of this invention finely divided powder of the inorganic material to be crystallized is delivered to a staionary torch which has a plurality of serially aligned powder feed tubes. Each of these powder feed tubes has associated with it a number of tube means essentially surrounding it and arranged to provide a combustible fueloxidant mixture around the powder feed tubes. The powder is thoroughly-mixed with the burning fuel-oxidant mixture and is fused before it strikes the boule. In the method and apparatus herein described the growth interface of the crystal is uniformly heated which in turn minimizes internal stresses and strains and optimizes the physical properties of the crystal so grown. Moreover, the distri bution of the fused powder on the boule surface is so controlled as to be essentially uniform over the entire surface. Uniformity of heating and of fused powder distribution are attained through the spacing of the powder feed tubes, the relationship between powder feed tubes and the means for delivering fuel and oxidant and the spacing between the torch and the boule growth surface. The boule itself is supported on a platform which moves only in a vertical plane.

The embodiment shown in FIGS. 1 through 5 is adapted for burning a gaseous hydrocarbon fuel with an oxidant such as air or oxygen; while the embodiment shown in FIGS. 6 and 7 is particularly suited for burning vaporized fuel oil and oxygen. As will be seen in the perspective, partially cut-away drawing of FIG. 1, the apparatus of this invention is comprised of a powder feeding mechanism 10, a torch 11 directed against the surface of a boule 12 which in turn is encased in a suitable insulation 13 and moved in a vertical plane on a support 14.

The finely divided powder which is to be fused to form the boule is contained within a canister 19 which termimates in a powder hopper 20v having as a bottom member a fine screen 21. The hopper 22 is so shaped as to correspond directly to the configuration of the powder feed tube holder 22, and in some instances it may be desirable to have the bottom edge of the hopper 20 fit inside the periphery of the powder feed tube holder 22. The powder canister 19 has a cover member 23 and associated with this cover is an anvil 24 held in a resilient mounting 25, an arrangement which allows the anvil to be tapped periodically by nonsparking hammer 27 supported on a pivot pin 28. By this meansthe fine powder is shaken through the screen 21 into the powder feed tubes. The means by which the hammer 27 is periodically raised and lowered is not an integral part of the invention and any suitable device may be used. For example, the hammer may be mounted on the shaft of a rotary solenoid. Any other device which is capable of periodically delivering the finely divided powder in nonagglomerated form to the powder feed tubes may also, of course, be used.

In the arrangement shown in FIG. 1 the oxidant, normally oxygen gas, is mixed with the powder prior to its being introduced into the gaseous fuel. Therefore, there is provided around the powder canister 19 a fluid-tight housing 35 which has a bottom member 36 and a top member 37. The chamber 38 which is defined by the housing completely surrounds the powder feed mechanism and is in fluid communication with an oxygen source (not shown) through an oxygen inlet line 39. Thus the oxygen is mixed with the fine powder and is carried along with it through the torch.

As will be seen from FIGS. 1 and 2, the bottom member 36 of the powder feed housing is attached through a resilient sealing gasket 46 by means of screws 40 to a torch-support plate and top member 45. The resilient gasket 46 may be eliminated and the bottom member 36 4 of the powder feed housing may be welded directly to the torch-support plate 45.

The powder feed tube holder 22 is constructed to pro-' vide a powder feed chute 51, formed of inclined side members 52 and 53 as shown in FIGS. 2 and 3. The serially aligned powder feed tubes 55 open into the powder feed chute 51 and they are held in spaced relationship by tube spacers 56. In order to insure the proper continuous and even feeding of the finely divided powder into all of the powder feed tubes 55, the interior of the powder feed mechanism has conically shaped or inclined members 57 which define powder-directing channels 58. Any means which insures the continuous and equal feeding of the finely divided powder into the plurality of aligned tubes 55' will, of course, be satisfactory for this purpose.

As will be seen in FIGS. 2 and 3, the torch itself is formed of an outer housing wall 60 which is divided internally by means of an upper tube mount member 61 and partially closed up by a lower tube support and bottom member 62.

Positioned around each of the powder feed tubes 55 are a number of fuel tubes 65 arranged, as will be seen in FIG. 4, to essentially surround each powder feed tube. In the embodiment of FIG. 2 these fuel tubes are positioned at a small angle with relation to the axis of the powder feed tube so that they touch at the bottom of the torch as will be seen in FIG. 4.

Associated with the upper portion of the outer torch housing wall 60 is an upper or first manifold housing 70 defining a chamber 71 which in turn communicates through multiple orifices 72 with a chamber 73 into which the upper ends of the fuel tubes 65 open. The gaseous fuel, e.g., a hydrocarbon such as propane or hydrogen, is introduced through fuel line 74 into chamber 73 to be conducted by means of the fuel tube 65 to e bottom of the torch where combustion takes place.

Positioned below the fuel manifold 70 is a second or oxidant manifold 78 defining a chamber 79 which communicates with the chamber 81 in the housing of the torch through multiple orifices 80. A suitable gaseous oxidant such as air or oxygen is introduced into chamber 63 through oxidant line 82.

Finally, around the bottom end of the housing 60 there is provided a third or bottom manifold which defines a chamber91 suitable for circulating a cooling fluid therein. Conduit means 92 are provided for introducing a cooling fluid into the chamber 91, and conduit means 93 are provided for withdrawing it.

As will be seen from FIG. 4, which is a bottom plan view of the torch, the housing wall 60 and the lower tube support and bottom member 62 define between them a rectangular opening 84 through which the powder feed tubes 55 and the fuel tubes 65 extend. Between these two types of tubes there are defined spacings 85 through which additional oxygen may, if desired, be introduced into the fuel-oxidant stream. If used, this oxygen is brought in by means of conduit 82 into chamber 81, and directly out and downwardly through the spacings 85 between the tubes. Thus, the combustible fuel mixture is made up of the fuel discharged by fuel tubes 65 and the oxidant which is mixed with the powder with any additional oxidant which may be discharged around the tubes from openings 85.

FIG. 3 shows a small fragmentary section of the top surface of the boule 12 and the deposition of the fused powder particles 94 in the form of a truncated cone 95 onto the boule surface. The spacing -96 between each of the powder feed tubes 55 and the distance d between the ends of the tubes 55 and the boule surface must be such that these cones 95 meet but do notexcessively overlap. (It should be noted, that the distance d in FIG. 3 is not properly scaled. Typically, it will be much greater than that shown, e.g., from 2 to 4 inches.) This insures a continuous and equal distribution of the fused particles on the boule surface and thus provides for the building up of the boule in a continuous even manner. In practice the powder streams at least partially diffuse into each other. The location of the fuel tubes around the powder feed tubes 55 to essentially suround each powder feed tube ensures complete fusion of the fine particulate material and uniform heating of the growth interface.

Turning back to FIG. 1, it will be seen that the boule 12 builds up in essentially plate-form 103 from a horizontally positioned rod-like seed crystal 102 which rests on the boule support. The seed crystal may, of course, take any other suitable form, for example a series of contiguous rods arranged to define the desired cross section of the crystal to be grown. The boule support, in turn, consists of a platform 100 and a vertical support rod 101 which is moved in a vertical plane by any suitable known means such as the vertical driving means described in copending application Ser. No. 646,523, filed in the names of Joseph F. Wenckus and Paul R. Doherty, and assigned to the same assignee as this application. In order to insure even and controlled cooling of the boule the boule support and the boule itself are positioned in a cavity 104 which is defined within an insulation 105. The insulation may, of course, have associated with it one or more windows for viewing the growth of the boule.

It is also within the scope of this invention to employ more than one serially aligned row of powder feed tubes along with additional fuel-oxidant supply means. FIG. 5 illustrates two parallel rows of feed tubes 55 with three rows of fuel tubes 65 and the spacings 85 defined between them used as oxygen feeding means. The apparatus of this invention lends itself to achieving any cross sectional configuration, provided the powder feed tubes are properly spaced and surrounded with fuel-oxidant delivery means. Hence, crystals with circular, rectangular, triangular or any other polygonal cross section may be grown.

The embodiment of the apparatus of this invention illustrated in FIGS. 6 and 7 is particularly suitable for burning oxygen or a gaseous oxidant with a vaporized fuel oil. In this figure like numbers refer to like elements as shown in FIGS. 1 through 5. It will be seen that in the embodiment of FIGS. 6 and 7 the housing 36 associated with the powder feed mechanism is welded directly to a torch support and mounting block 110 thus eliminating the resilient gasket sealing means shown in FIG. 1. Sealed to the torch support 110 and depending therefrom is a fluid-tight housing 111 which defines within it a mixing chamber 112. The torch which is positioned at the bottom end of the housing 111 comprises a sleeve 116 which fits into the housing 111 and which in turn is closed with a torch block 117. Into the torch block are drilled a number of channels 118 which essentially surround each of the feed tubes 55 as will be seen from the bottom cross section in FIG. 7. The channels 118 communicate with smaller-diameter fuel passages 119 which deliver the mixture of fuel and oxidant to the bottom face of the torch where ignition takes place and surrounds the powder feed tube 55 with hot combustion gases.

The fuel oil may be vaporized by any suitable means such as disclosed in copending U.S. Ser. No. 7 10,915, now U.S. Pat. 3,477,644, filed in the names of Leon M. Bablouzian, Roger A. Castonguay and Joseph F. Wenckus, and assigned to the assignee of the present invention. The vaporized fuel oil is introduced through fuel line 120 into the mixing chamber 112. A suitable gaesous oxidant, such as oxygen, previously preheated if desired, is brought in through conduit 121 into the bottom portion of the mixing chamber 112 through a small orifice 122. This arrangement sets up turbulent conditions within the mixing chamber 112 and insures complete mixing of the oxidant with the vaporized fuel prior to its introduction into fuel channels 118 and then into fuel passages 117. Thus, as

will be seen in FIG. 7, there is provided another meansfor essentially surrounding each powder feed tube 55 with a combustible fuel/oxidant mixture which in burning generates suflicient heat to fuse the fine powder particles and to uniformly heat the growth interface of the boule. The torch of FIGS. 6 and 7 may also, of course, be constructed to provide more than one line or array of powder feed tubes as discussed in conjunction with FIG. 5.

It is also within the scope of this invention to form an assembly of torches with separate or a common powder feed means. An example of such an assembly is illustrated diagrammatically in FIG. 8 where 5 torches 11a- 11e are aligned or locked together to produce a wide plate crystal. In such an arrangement the spacing between the exit ends of the tubings should be maintained as small as possible in order to achieve an essentially even distribution of fused powder over the growth interface of boule 103. The arrangement in FIG. 8 may have certain advantages over a single torch means of a length equal to the total lengths of the five components of FIG. 8. Thermal losses from the ends of the plate crystal 103 may be con- Siderably greater than those experienced by the central portion of the crystal. The use of separate torch means permits adjusting and controlling the temperatures in the various torches so as to achieve an even temperature along the length of the crystal and thus to minimize stresses and strains.

The method and apparatus of this invention are adapted to growing crystals of any inorganic material or mixture of materials which lends itself to crystal formation th'rough flame fusion. The fuel mixture, the feed rate and other operational parameters will desirably be chosen to optimize crystal formation by this technique. Generally, the materials used will be refractory oxides and mixtures thereof with or without small quantities of impurities, i.e., chromic oxide in aluminum oxide to form ruby crystals.

As an example of the operation of this apparatus, a sapphire blank was grown which was about two and onehalf inches wide and one inch long and about one-half inch thick. The seed crystal used was a rod approximately one-eighth inch in diameter and about two and one-half inches long, and the growth interface was maintained between about two and four inches from the delivery end of the powder feed tubes. Crystal growth was at the rate of about one inch per hour. This growth rate may vary from one-fourth to one and one-half inches per hour, depending upon the material used, and the rate of powder feed. The resulting crystal blank formed can be cut and polished with a minimum of breakage loss.

It will thus be seen that this invention provides a method and apparatus for readily forming single crystals in plate or sheet form at a cost far below that now encountered.

I claim:

1. An apparatus for growing crystal blanks of inorganic materials which can be flame fused and deposited on a crystal growth interface, comprising in combination (a) an array of a plurality of powder feed tubes aligned in the desired cross sectional configuration of the crystal to be grown and maintained in spaced relationship within a single housing, said powder feed tubes opening at their upper ends into a single common powder feed chute;

(b) powder delivery means for delivering finely divided powder into said powder feed chute and to said upper ends of said powder feed tubes, said powder delivery means including conically configured powder directing means positioned within said chute between said upper ends of said powder feed tubes and adapted to introduce powder uniformly into the inlet ends of said tubes;

(c) means for providing a combustible gaseous fueloxidant mixture essentially encircling each of said powder feed tubes;

((1) boule support means directly below the discharge ends of said array of powder feed tubes; and

(e) boule support moving means adapted to move said boule support in a vertical plane only; the spacing between the powder feed tubes at their discharge ends and the distance from their discharge ends to the growth interface of the crystal being grown be- (1) means for mixing a portion of said oxidant with said powder,

(2) fuel tube means surrounding said powder feed tubes, and

(3) means for delivering the remainder of said oxidant around the discharge ends of said powder feed tubes and said fuel tubes.

4. An apparatus in accordance with claim 1 wherein said means for providing a combustible gaseous fuelmixture comprises in combination (1) a plurality of small-diameter fuel-oxidant passages positioned around each powder feed tube, and

(2) means to supply said fuel-oxidant mixture to said passages.

5. An apparatus for growing crystal blanks of inorganic material in plate from which can be annealed, cut and polished, comprising in combination a plurality of contiguous aligned means for depositing flame fused inorganic particles on a crystal growth interface, each of said means comprising (a) a plurality of serially aligned powder feed tubes maintained in spaced relationship within a single housing, said powder feed tubes opening at their upper ends into a single common powder feed chute;

(b) powder delivery means for delivering finely divided powder into said powder feed chute and to said upper ends ofsaid powder feed tubes, said powder delivery means including conically configured 8 powder directing means positioned within said chute between said upper ends of said powder feed tubes and adapted to introduce powder uniformly into the inlet ends of said tubes;

(c) means for providing a combustible gaseous fueloxidant mixture essentially encircling each of said powder feed tubes; and

(d) boule support means movable in a vertical plane only and located directly below the discharge ends of said powder feed tubes; the spacing between the powder feed tubes at their discharge ends and the distance from their discharge ends to the growth interface of the crystal being grown being so adjusted that the fused powder is substantially evenly distributed over said interface and said interface is substantially uniformly heated, whereby said crystal is formed in a manner to minimize strains and stresses.

6. An apparatus in accordance with claim 5 wherein a single means for delivering said finely divided powder is provided for all of said means for depositing said flame fused particles, and separate means for providing said combustible gaseous fuel-oxidant mixture are provided for at least the two terminal contiguous aligned means.

References Cited UNITED STATES PATENTS 2,692,456 10/1954 Dauncey 23-273 V 3,077,752 2/1963 Drost et a1 23-273 V 3,224,840 12/1965 Lefever '23-273 V FOREIGN PATENTS 1,269,105 2/1962 France.

NORMAN YUDKOFF, Primary Examiner S. SILVERBERG, Assistant Examiner US. Cl. X.R. 23301 SP 

